This kind of current resonant type switching power supply device is configured as depicted in FIG. 5. The switching power supply device includes a full-wave rectifier circuit 101 that rectifies a commercial AC, a power factor correction type first converter 102 to which an AC full-wave rectified output of the full-wave rectifier circuit 101 is input, and a current resonant type second converter 103 to which a DC output of the first converter 102 is supplied.
The second converter 103 is an LLC current resonant type converter, and includes an isolation transformer 104 having a primary winding L1 and a secondary winding L2, a capacitor 105 connected to the primary winding L1 of the isolation transformer 104 to form an LLC resonant circuit with a leakage inductance of the isolation transformer 104, a half-bridge circuit 106 having two switching elements connected to the winding L1 of a primary side of the isolation transformer 104, and a power supply control circuit 107 that drives the switching elements forming the half-bridge circuit.
Herein, for example, as described in JP 5384973 B, the power supply control circuit 107 includes a setting means that sets a normal mode for performing power supply control by continuously operating an oscillator and a burst mode for performing power supply control by intermittently operating the oscillator and a burst operation setting means that detects an output voltage of a secondary side of the isolation transformer when the burst mode is set by the setting means, in which the burst operation setting means starts a switching pulse oscillation operation upon reduction of the output voltage of the secondary side, and stops the switching pulse oscillation operation upon return of the output voltage of the secondary side.
FIG. 6 is a simplified illustration of a circuit structure of the burst operation setting means. Specifically, a burst operation setting circuit 111 includes a feedback voltage input terminal tFB2 to which a feedback voltage is input from the secondary winding L2 side of the isolation transformer and a standby command input terminal tSTB2 to which a standby command signal input from an unillustrated standby command circuit is input via a photocoupler 112.
Additionally, the burst operation setting circuit 111 includes a comparator 113, to a non-inverting input terminal of which a feedback voltage VFB2 input to the feedback voltage input terminal tFB2 is input and to an inverting input terminal of which a threshold voltage Vref1 is input, a constant current circuit 114 that supplies a constant current to the standby command input terminal tSTB2, an inverter 115 to which a connection point between the constant current circuit 114 and the standby command input terminal tSTB2 is connected and which performs sign inversion, and an OR circuit 116 to which an output of the comparator 113 and an output of the inverter 115 are input.
When an output of the OR circuit 116 is at a high level, the power supply control circuit 107 goes into a switching operation state, and when at a low level, the power supply control circuit 107 goes into a switching stop state. When the normal mode is set by a signal from outside, a phototransistor forming the photocoupler 112 is turned on, causing the input of the inverter 115 to go to a low level. As a result of that, the output of the inverter 115 and the output of the OR circuit 116 go to a high level, thereby causing the power supply control circuit 107 to go into the switching operation state. On the other hand, when the burst mode is set by a signal from outside, the phototransistor forming the photocoupler 112 is turned off, causing the input of the inverter 115 to go to a high level. As a result of that, since the output of the inverter 115 goes to a low level, whether or not the power supply control circuit 107 goes into the switching stop state is determined by a magnitude correlation between the feedback voltage VFB2 and the threshold voltage Vref1.